When highly pathogenic avian influenza (HPAI) H5N1 spread rapidly from Asia to Europe and Africa during 2005-2006, the finger was immediately pointed at wild migratory birds as potential sources of the spread of this disease. HPAI H5N1 is primarily a disease of poultry, and infection within poultry flocks typically results in mass mortality (Alexander 2007). However, HPAI H5N1 can also infect other species, including wild birds and humans (Chen et al. 2006). The potential risks to human health resulted in extensive media coverage,much of which focused on wild migratory birds despite virtually all cases of human infection being linked to prolonged and close contact with poultry. The focus of the media coverage on migratory birds fuelled public concerns,resulting in calls for mass culling of wild birds, drainage of wetlands and other such responses, especially following reports of mortality events among several migratory wildfowl species (Hesterberg et al. 2009).

Wild migratory birds are clear candidates for the spread of such zoonotic pathogens, and information on the migratory movements of many species was rapidly assessed in attempts to identify potential routes and timings of spread of the disease (Chen et al. 2006; Yasue et al. 2006; Feare 2007; Gauthier-Clerc, Lebarbenchon & Thomas 2007). However, the actual risk of HPAI H5N1 spread through migratory birds clearly also depended on whether infected individuals were capable of migratory movements, and the distance over which such individuals could travel. Answering these questions required much more detailed analyses of patterns of symptomatic and asymptomatic infection, and migratory routes and timings for individuals of a range of species.

In this issue’s Editor’s Choice, Gaidet et al. (2010) report the findings of an exhaustive study of satellite tracking of individual migratory birds, and a review of experimental studies of the apparent duration of asymptomatic infection of such species. Through collation of telemetry tracks of 228 individuals of 19 species throughout Europe, Asia and Africa, they evaluate the dispersive potential of HPAI H5N1 by migratory wildfowl. The study focuses on wildfowl (ducks, geese and swans) because HPAI H5N1 infection has been found in apparently healthy free-living wildfowl (Hesterberg et al. 2009), and the extensive flocking and migratory behaviour of many wildfowl species provides potential for transmission of the disease across national borders. Gaidet et al.’s review reveals that all the wildfowl species included in the study are receptive to HPAI H5N1 infection and show a period of asymptomatic infection and viral shedding. In addition, the satellite telemetry studies indicated that these species could, in theory, move distances of several hundred to a few thousand kilometres within the period of asymptomatic infection.

However, virus dispersal would require asymptomatic infection to coincide with migration, thus the authors used the duration of migratory movements to estimate the actual number of days during which an infected individual could potentially disperse the virus over long distances. These estimates show that there are in fact only around 5 to 15 days per year during which dispersal of the virus over 500 km could occur, and that intercontinental disease transmission by migratory wildfowl is highly unlikely because most wildfowl stop on migration at staging posts for periods longer than the asymptomatic duration period. The authors also highlight the fact that these analyses assume that asymptomatically infected birds will attempt to migrate, which may be unlikely if activation of the immune system to tackle the infection increases the energetic costs to those individuals.

Such detailed studies of the risks associated with the spread of zoonotic diseases by migratory birds are extremely important, as they can provide key information to the public and relevant authorities, and avoid the ill-informed panic that characterised much of the media response to avian influenza. Satellite tracking of these species took place through an international programme, co-ordinated by the FAO, highlighting the potential for collaborative, international research to provide rapid and effective information relating to important, emerging issues. HPAI H5N1 has now been reported from more than 60 countries (WHO 2010), and the information reported in this study should hopefully help to focus attention on the role of human activities in facilitating spread of this disease, and to reduce public concerns over wild migratory birds.